Paul.foot, you have a Victa 18 Model 5 series 1, it looks to have the right engine as the number sugests and is quite early in the series, I would guess 1958, and as Deejay has pointed out the boss will need to be removed to seperate the cases, so remove the barrel and rest the boss on a substantial block of steel like a good heavy vise (I use a bit of railway iron for an anvil) or similar, then give it a good wack with a decent hammer and it will pop off the tapered shaft, (similar to cracking ball joints) so to try and make myself a bit clearer, is what you are doing is hitting the side of the boss, not trying to knock the boss off the end of the shaft, I hope you can work out my babblings, good luck with the mower.
